222 PROCEEDINGS OF SECTION C. 
expelled from the magma, and penetrates into the surround- 
ing rock. In Tasmania we have examples of the former 
class in the diabase (dolerite) which occurs in such 
abundance over a large part of the Island; whilethe granite 
and gabbros on the West, North, and East Coasts are 
representatives of the latter. It is a significant fact that 
wherever the granite and gabbro occur in Tasmania, we 
also find numerous ore deposits, whereas not a single deposit 
has yet been shown to have any connection with the diabase. 
Contact metamorphism often takes place without con- 
siderable migration of material from the eruptive magma 
to the rigid neighbouring rocks, and in this case the mole- 
cules are merely re-arranged into new compounds under the 
influence of the injected steam. In other cases, however, 
a considerable migration of material takes place, as, for 
example, in the conversion of beds of limestone into garnet 
rock, epidote, &c. Sometimes the migration of certain 
metals is so great that very considerable ore deposits are 
formed. 
There is no doubt that some deposits may have been 
produced directly by sublimation. The small deposits of 
specularite which are formed in the crevices of recent lavas 
and in the craters of active volcanoes are evidence of this. 
These are true sublimates formed after the reaction :— 
2 Fe Cl, + 3 H,O = Fe,O, + 6H CL 
After Bensen, the process by which minerals are formed 
by reactions of gases on one another is known as pneumato- 
lysis. In the metallurgical processes of roasting and smelt- 
ing, products of sublimation are often formed which 
possess the same .characters as natural ores. As, for 
example, artificial galena, zinc blende, realgar, &c. Besides 
these, a whole series of minerals have been artificially pro- 
duced by experimentalists in pneumatolysis. 
Contact Metamorphic Deposits. 
This class of deposit we may regard as having been 
scientifically proved to have derived its metallic contents 
directly from eruptive rocks. After Von Groddeck,* these 
deposits are known as the Christiania type, for they occur 
in large numbers in the Christiania district in Norway. 
They are now known to exist in almost all countries, but in 
proportion to their distribution they have not proved of 
very great commercial importance. 
. Lehre von die lagerstatten der Erze, 1878, p. 260. 
